Abstract: Apparatus for monitoring the time duration in which a visitor remains in a defined region on the way from a visited place to another place, more particularly from the entry to the visited place and back, freely and without supervision. At the entry place, the visitor receives a transmitter with a timing member. After termination of a given time duration which corresponds to the normal travelling time of the visitor between the visited places, the transmitter transmits an individual identification signal which is received by a central receiver and is decoded. It can be immediately ascertained which visitor dwells for too long a time within the defined region so that searching or controlling measures can be initiated.

Abstract: The invention relates to a circuit arrangement having several signal paths which can be activated by a switchable current source. For the current source, use is made of I.sup.2 L gates whose injector connections are combined in two groups. When a switch-over is made from one signal path to another, a decreasing current is applied to the injector connection of the I.sup.2 L gate connected to one signal path while an increasing current is applied to the corresponding connection of the I.sup.2 L gate associated with the other signal path.

Abstract: A treble-control circuit which exhibits a particularly low noise level and which can be controlled digitally in a simple manner. The treble control circuit comprises an amplifier having an output connected to the arrangement of a capacitor in series with a resistor chain. The tappings of said resistor chain are connected to an inverting input of the amplifier via a first switch and to the output of the treble control circuit via a second switch.

Abstract: A low-noise bass-control device, which is digitally controllable in a simple manner, comprises an amplifier having an output connected to the series arrangement of a capacitor and a tapped resistor chain. The taps on said resistor chain are connected to an inverting input of the amplifier via a first switch and to the output of the control device via a second switch.

Abstract: An amplifier comprising at least one emitter-coupled transistor pair with a signal source connected in the emitter line. Amplifier non-linearities mainly caused by the second harmonic are compensated by supplying the base of at least one of the transistors with a compensation signal of suitable phase that is derived from either the input or the output signal of the amplifier. This results in a multiplication process which produces a second harmonic in the collector current of the two transistors which, with the correct phase of the compensation signal at the input, exactly compensates the second harmonic caused by the amplifier non-linearity. If the transistor pair is used for gain or volume control, the direct voltage between the bases of the two transistors, and the compensation signal, will both be varied.

Abstract: The circuit arrangement in accordance with the invention comprises an RC-element, to which the same direct voltage is applied as the amplifier. When the direct voltage is switched on or off the capacitor is either charged or discharged, the resulting oppositely directed voltages being applied to the anti-parallel base-emitter junctions of two transistors, whose collectors are interconnected. The collector voltage of these transistors serves to control a semiconductor switch included in the audio frequency branch.